The invention relates to a method for operating a Diesel engine with a pump-line-nozzle injection system including means for controlling the fuel pressure in the pump-line-nozzle system.
Diesel engines operating with pump-line-nozzle injection systems are known in the art. Because each individual cylinder has its own injection unit comprising a pump, a fuel supply line, a nozzle and a pump which is driven by the camshaft of the internal combustion engine and is arranged close to the nozzle, the fuel supply lines are only short and can withstand very high injection pressures well into the range of 1000 bar and above. This is achieved with a design, in which the highest injection pressure occurs at the rated speed and full load, for which the design parameters of the system such as cam velocity, diameter of the pump plunger and flow cross-section of the injection nozzle are chosen and which are critical in this respect. At lower engine speeds and lower engine loads, the injection pressures fall substantially. The pressure profile obtained over the engine speed is roughly triangular with a flank that rises more gently toward the maximum pressure.
Pumps for pump-line-nozzle injection systems of this kind are known for example from WO 97/01 031. In this known pump design, the pump is provided with a delivery-side control valve that allows a modification of the shape of the pressure profile curve to lower the pump pressure by discharging fuel to the low-pressure side.
It is the object of the present invention to provide a method for operating a Diesel engine with a pump-line-nozzle injection system by which soot and particulate emissions of the Diesel engine are reduced, especially in the intermediate and lower engine-speed and load range.
In a method for operating a Diesel engine with a pump-line-nozzle injection system, which allows the pressure level to be raised in the lower and intermediate engine speed range in relation to a predetermined maximum permissible injection pressure without this maximum permissible injection pressure being exceeded in the upper engine speed range by providing an injection pump designed to deliver the desired fuel injection pressure at low and intermediate engine speeds and including a pressure relief valve in the fuel injection line permitting limiting of the fuel injection pressure at higher engine speeds.
With the present invention, the level of the engine speed-dependent pressure is increased in the intermediate and lower engine speed and load range, in particular by designing the system for a theoretical pressure point that is above the rated speed/full load point and the corresponding maximum permissible injection pressure. In this way a pressure profile curve is obtained, whose level is higher overall, but which is lowered by opening the control valve in the region of critical values, i.e. when or before the maximum permissible injection pressure is exceeded.
With the method according to the invention, a relatively high degree of flexibility is achieved in the definition of the pressure profile curve without the need for changing the mechanics of the system. The disadvantages associated normally with a design providing for a higher-pressure level, including that of a higher power requirement, do not take effect. In the lower engine speed ranges, the increase in pressure is desired and mechanically acceptable and in the critical upper speed range, the pressure is limited because excess pressure is released through a short circuit to the low-pressure side. As a result, there is no significant rise in the power requirement apart from pumping losses.
Within the context of the invention, it is possible to adjust the pressure profile even before thy pressure corresponding to the maximum permissible injection pressure is reached if a pressure increase is desired only in the lower speed range.
Further details and features of the invention will become apparent from the following description of one exemplary embodiment of the invention described below with reference to the accompanying drawings: